The presence of relatively high levels of petroleum acids, e.g., naphthenic acids, in crude oils or fractions thereof is a problem for petroleum refiners and more recently for producers as well. Essentially, these acids, which are found to a greater or lesser extent in virtually all crude oils, are corrosive, tend to cause equipment failures, and lead to high maintenance costs, more frequent turnarounds than would otherwise be necessary, reduce product quality, and cause environmental disposal problems.
A very significant amount of literature, both patents and publications, exists that deal with naphthenic acid removal by conversion or absorption. For example, many aqueous materials can be added to crudes or crude fractions to convert the naphthenic acids to some other material, e.g., salts, that can either be removed or are less corrosive. Other methods for naphthenic acid removal are also well known including absorption, on zeolites, for example. Additionally, one common practice for overcoming naphthenic acid problems is the use of expensive corrosion resistant alloy materials in refinery or producer equipment that will encounter relatively high naphthenic acid concentrations. Another common practice involves blending of crudes with high TAN with crudes of lower TAN, the latter, however being significantly more costly than the former. One reference, Lazar, et al (U.S. Pat. No. 1,953,353) teaches naphthenic acid decomposition of topped crudes or distillates, effected at atmospheric pressure between 600 and 750.degree. F. (315.6 to 398.9.degree. C.). However, it only recognizes CO.sub.2 as the sole gaseous non-hydrocarbon, naphthenic acid decomposition product and makes no provision for avoiding buildup of reaction inhibitors.
Additionally, U.S. Pat. No. 2,921,023 describes removal of naphthenic acids from heavy petroleum fractions by hydrogenation with a molybdenum oxide-on-silica/alumina catalyst. More specifically, the process preferentially hydrogenates oxo-compounds and/or olefinic compounds, for example, naphthenic acids, in the presence of sulfur compounds contained in organic mixtures without affecting the sulfur compounds. This is accomplished by subjecting the organic mixture to the action of hydrogen at temperatures between about 450 and 600.degree. F. (232.2 to 315.6.degree. C.), in the presence of a molybdenum oxide containing catalyst having a reversible water content of less than about 1.0 wt %. Catalyst life is prolonged by regeneration.
WO 96/06899 describes a process for removing essentially naphthenic acids from a hydrocarbon oil. The process includes hydrogenation at 1 to 50 bar (100 to 5000 kPa) and at 100 to 300.degree. C. (212 to 572.degree. F.) of a crude that has not been previously distilled or from which a naphtha fraction has been distilled using a catalyst consisting of Ni-Mo or Co-Mo on an alumina carrier. The specification describes the pumping of hydrogen into the reaction zone. No mention is made of controlling water and carbon dioxide partial pressure.
U.S. Pat. No. 3,617,501 describes an integrated process for refining whole crude but does not discuss TAN reduction. The first step of the process includes hydrotreating a feed, which can be a whole crude oil fraction, using a catalyst comprising one or more metals supported on a carrier material. Preferably the metals are metal oxides or sulfides, such as molybdenum, tungsten, cobalt, nickel and iron supported on a suitable carrier material such as alumina or alumina that contains a small amount of silica. The catalyst can be employed in the form of fixed bed, a slurry or fluidized bed reactor. With regard to slurry operation, no mention is made of catalyst particle size, catalyst concentration in feed or the use of unsupported catalysts (i.e., no carrier).
British Patent 1,236,230 describes a process for the removal of naphthenic acids from petroleum distillate fractions by processing over supported hydrotreating catalysts without the addition of gaseous hydrogen. No mention is made of controlling water and carbon dioxide partial pressure.
U.S. Pat. Nos. 4,134,825; 4,740,295; 5,039,392; and 5,620,591, all of which are incorporated herein by reference, teach the preparation of highly dispersed, unsupported catalysts, of nominal particle size of one micron, from oil soluble or oil dispersible compounds of metals selected from groups IVB, VB, VIB, VIIB and VIII of the periodic table of elements and application of said catalysts for the hydroconversion upgrading of heavy feeds, including whole or topped petroleum crudes. Hydroconversion is defined in these patents as a catalytic process conducted in the presence of hydrogen wherein at least a portion of the heavy constituents and coke precursors (i.e., Conradson Carbon) are converted to lower boiling compounds. The broadest ranges cited in these references with respect to process conditions include temperatures in the range of 644-896.degree. F. (339.9 to 480.degree. C.) hydrogen partial pressures ranging from 50-5000 psig (446.08 to 34576.33 kPa) and from 10-2000 wppm of catalyst metal based on the weight of the feedstock. These references are directed to the conversion upgrading of heavy feeds and do not recognize that said catalysts can be used to selectively destroy carboxylic acids, e.g., naphthenic acids.
Another method for removal of such acids includes treatment at temperatures of at least about 400.degree. F. (204.44.degree. C.), preferably at least about 600.degree. F. (315.56.degree. C.) while sweeping the reaction zone with an inert gas to remove inhibitors indigenous to or formed during the treatment. However, this approach is debited by the volatilization of some of the naphthenic acids, which are found in distillate and light oil fractions that flash during the thermal treatment. Moreover, treatment temperatures may be too high for this method to be used in downstream applications where it is desirable to destroy the acids prior to pipestill furnaces, i.e., at temperatures of about 550.degree. F. (287.78.degree. C.) or below.
Thus, there remains a need for eliminating or at least substantially reducing petroleum acid concentration in crudes or fractions thereof that is low cost and refinery friendly. Such technology would be particularly suitable for crudes or fractions where the TAN value is about 2 or above. TAN, determined by ASTM method D-664, is milligrams of KOH required to neutralize the organic acids contained in 1.0 gram of oil.